The Coronae of F-K Dwarfs: Factors Controlling Their Properties
Session 38 -- EUVE -- Astronomy II
Oral presentation, Tuesday, 2:00-6:30, Zellerbach Auditorium Room

## [38.06] The Coronae of F-K Dwarfs: Factors Controlling Their Properties

C. Jordan (U. of Oxford)

X-ray observations of late-type stars can be used to determine average coronal emission measures, Em(T$_c$), and temperature, T$_c$, provided the spectra can be fitted adequately by single temperatures. Em(T$_c$) and T$_c$ can be correlated with stellar properties, such as the rotation period, or with convection zone parameters, such as the Rossby number (Ro = P(rot)/turnover time at the base of the convection zone). Including F to K dwarfs, the closest correlations are found between Em(T$_c$) g$\cdot ^{\frac{1}{2}}$ and Ro, and T$_c$ g$\cdot ^{\frac{1}{2}}$ and Ro. The total energy losses from the coronae (radiation plus thermal conduction) can, with some assumptions, also be expressed in terms of Em(T$_c$) and T$_c$. These total losses, and hence the non-thermal energy required to heat the corona, scale approximately as Ro$^{-1}$. The coronal magnetic field can also be expressed in terms of Em(T$_c$) and T$_c$, through the pressure and plasma Beta, and comparisons can be made with specific heating mechanisms. Some recent results will be presented, including the relation between these implied coronal fields and surface magnetic fields and filling factors. The contribution from new observations with ROSAT, and expected from EUVE, will be discussed.

This work has been supported by the SERC through grant GR/H25539.